The present invention relates generally to electric or hybrid electric vehicles. More specifically, the present invention relates to an electric or hybrid electric vehicle (HEV) having regenerative and friction brakes that utilize wheel slip data of the regeneratively braked axle to selectively tortionally connect a second axle with the first axle to extend the range of regenerative braking.
The general principle of regenerative braking is recognized by manufacturers of electric and HEVs as a way to increase the overall efficiency of the vehicle. Regenerative braking seeks to recover the kinetic energy of the vehicle which is normally dissipated as heat through a normal hydraulic friction brake system, by operating the electric motor drive as a generator to restore the generated electricity to a battery or other energy storage device. However, regenerative braking has certain limitations. The maximum amount of regenerative braking torque available is not a constant, but is a function of a normal force at a tire""s patch and a rolling surface coefficient of friction. If the regenerative brake torque demand exceeds the physics of this interface, wheel slip will be triggered. Most antilock braking systems (ABS) require that the brakes be applied and released on a given wheel at a rate that exceeds the frequency response of the typical electric motor. Many ABS require that the friction brakes be applied and released on a given wheel approximately a minimum of seven times a second. Accordingly, during an ABS incident, most electric and HEVs are programmed to shut off the regenerative braking and begin the friction braking. Shutting off the regenerative braking causes energy losses and a lowering of overall fuel economy. It would therefore be advantageous to provide a method of braking for an electric or a HEV wherein the incidents which require a termination of regenerative braking during an ABS mode of operation be minimized. This and other issues related to electric and HEVs are the subject of the following U.S. Pat. Nos. 4,962,969; 5,269,390; 5,294,191; 5,358,084; 5,378,053; 5,421,643; 5,450,324; 5,551,764; 5,573,312; 5,615,933; 5,632,534; 5,895,100; 6,070,689; 6,076,899; 6,086,166; and 6,099,089.
In a preferred embodiment of the present invention, when a regenerative braking situation occurs such as in a lift throttle or operator-signaled brake command, a motor generator regeneratively brakes the driven wheels of a primary driving axle of the vehicle. If a slip condition occurs wherein the driven wheels are locked by application of the regenerative brakes, a clutch which torsionally connects the secondary axle to the primary axle is engaged. Regenerative braking will then occur over all four wheels of the vehicle, effectively cutting in half the friction requirement for the primary driven wheels. Accordingly, in many cases the primary driven wheels will become unlocked and the ABS operation will not be required. Not using the ABS mode of operation allows regenerative braking to be maintained and accordingly, the fuel economy of the vehicle is enhanced. When the regenerative braking operation is over, the clutch between the two axles may be opened and the vehicle can continue onward in a two-wheel drive mode of operation.
An advantage of the present invention is to provide a method of regenerative braking for a four-wheel drive electric or HEV which also has conventional brakes for an antilock mode of operation.
A further advantage of the present invention is to provide a four-wheel drive vehicle which can stay in a regenerative braking operation for longer periods of time than previously possible.
Other advantages of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.